Superconducting Sr2RuO4 Thin Films without Out-of-Phase Boundaries by Higher-Order Ruddlesden–Popper Intergrowth

Ruddlesden-Popper (RP) phases ( A n +1 B n O 3 n +1 , n = 1, 2,···) have attracted intensive research with diverse functionalities for device applications. However, the realization of a high-quality RP-phase film is hindered by the formation of out-of-phase boundaries (OPBs) that occur at terrace edges, originating from lattice mismatch in the c -axis direction with the A ' B 'O 3 ( n = ∞) substrate. Here, using strontium ruthenate RP-phase Sr 2 RuO 4 ( n = 1) as a model system, an experimental approach for suppressing OPBs was developed. By tuning the growth parameters, the Sr 3 Ru 2 O 7 ( n = 2) phase was formed in a controlled manner near the film-substrate interface. This higher-order RP-phase then blocked the subsequent formation of OPBs, resulting in nearly defect-free Sr 2 RuO 4 layer at the upper region of the film. Consequently, the Sr 2 RuO 4 hin films exhibited superconductivity up to 1.15 K, which is the highest among Sr 2 RuO 4 films grown by pulsed laser deposition. This work paves the way for synthesizing pristine RP-phase heterostructures and exploring their unique physical properties.